Die-casting is an important metallurgical process, which produces geometrically complex near net shaped metallic parts with excellent surface finish at a low scrap rate and high production rate. Due to the severe mechanical and thermal cyclic loading, die failure is a significant issue in die-casting that results in considerable energy consumption and wastage. It is therefore important for a die caster to delay the die failure as much as possible by adopting suitable protective measures.
Die surfaces may be protected by covering the active surface by a multilayered engineered coating that accommodates the thermal fatigue and provides good wear resistance. There has been significant progress in the die coating research that has resulted in the patents such as U.S. Pat. Nos. 6,333,103 to Ishii et al.; 5,766,782 and 5,851,687 to Ljungberg; and 7,096,921 to Moore. With the innovations disclosed in these patents, recent years have witnessed an extension of the die life by 3-100 times.
The severe conditions of die casting process still have a long-term detrimental effect on the properties of the die-protecting layer. The prolonged operation at these conditions can initiate a failure in the die coating, which can rapidly extend to the main die surface, again causing failure.
It is seen that, once severe cracking occurs in the coating, the die needs to be removed from production and the coating can be etched (removed) from the die surface by electrochemical means and a fresh coating can be deposited on the same die thereby allowing it to be continually used for much longer times. The repair, however, is possible only when the failure is largely confined to the protective layer, without greatly propagating inside the actual tool steel die surface (substrate). The important issue, therefore, is to determine the point at which the protective surface coating approaches failure and indicates the need for its change.
This is why we intend to incorporate a ‘smart’ die coating system which is based on the development of a piezoelectric thin film sensor embedded in the optimized ACSEL coating (or any other coating system). In 1987, Ramalingam et al. from the University of Minnesota filed and obtained patent (U.S. Pat. No. 4,927,300) on the successful use of embedded piezoelectric sensors to measure the transient stress within a WC lathe machine part that is often subjected to periodic impulsive forces. The sensor operated at room temperature and generated periodic signals that were synchronous with the time varying force to which the tool is subjected. This invention was of more importance for monitoring unmanned machining systems in a production line and to optimise the machining production and performance.
Although technological advances have led to an extension of die life, there is still little technology available to predict structural problems with dies. To date, the problem of die wear monitoring is accomplished by observation of the die cavity and quality of the castings produced. It is a ‘post mortem’ procedure that does not allow the die caster to become aware of the problem until the problem is irreversible.